[patch 038/134] mm/filemap.c: don't initiate writeback if mapping has no dirty pages

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From: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Subject: mm/filemap.c: don't initiate writeback if mapping has no dirty pages

Functions like filemap_write_and_wait_range() should do nothing if inode
has no dirty pages or pages currently under writeback.  But they anyway
construct struct writeback_control and this does some atomic operations if
CONFIG_CGROUP_WRITEBACK=y - on fast path it locks inode->i_lock and
updates state of writeback ownership, on slow path might be more work. 
Current this path is safely avoided only when inode mapping has no pages.

For example generic_file_read_iter() calls filemap_write_and_wait_range()
at each O_DIRECT read - pretty hot path.

This patch skips starting new writeback if mapping has no dirty tags set. 
If writeback is already in progress filemap_write_and_wait_range() will
wait for it.

Link: http://lkml.kernel.org/r/156378816804.1087.8607636317907921438.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 mm/filemap.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/mm/filemap.c~mm-filemap-dont-initiate-writeback-if-mapping-has-no-dirty-pages
+++ a/mm/filemap.c
@@ -408,7 +408,8 @@ int __filemap_fdatawrite_range(struct ad
 		.range_end = end,
 	};
 
-	if (!mapping_cap_writeback_dirty(mapping))
+	if (!mapping_cap_writeback_dirty(mapping) ||
+	    !mapping_tagged(mapping, PAGECACHE_TAG_DIRTY))
 		return 0;
 
 	wbc_attach_fdatawrite_inode(&wbc, mapping->host);
_



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